UHV Knowledge | Characteristics and Functions of Converter Station Equipment

A converter station is a system that enables the mutual energy conversion between direct current (DC) and alternating current (AC) in a DC transmission project. In addition to equipment such as the AC yard, which is the same as that in an AC substation, a DC converter station also has the following unique equipment: converters, converter transformers, AC-DC filters and reactive power compensation equipment, and smoothing reactors.

The main function of a converter is to carry out the conversion between AC and DC. Evolving from the initial mercury arc valve to the electronically controlled and optically controlled thyristor valves, the unit capacity of the converter has been continuously increasing.

The converter transformer is a key equipment for the AC-DC conversion in a DC converter station. Its grid side is connected to the AC yard, and its valve side is connected to the converter. Therefore, the valve-side winding of the converter transformer needs to withstand the combined stress of AC and DC. Since the operation of the converter transformer is closely related to the nonlinearity caused by the commutation of the converter, it has different characteristics from ordinary power transformers in terms of leakage reactance, insulation, harmonics, DC bias magnetism, on-load tap changing, and testing.

The AC-DC filters provide a passage to the ground for the characteristic harmonics generated during the operation of the converter. A large number of harmonics are generated during the operation of the converter, consuming 40% to 60% of the reactive power of the converter capacity. The AC filters not only filter the harmonics but also provide reactive power. When the reactive power provided by the AC filters is insufficient, special reactive power compensation equipment is also required.

The smoothing reactor can prevent lightning and steep waves on the DC side from entering the valve hall, thus protecting the converter valves from the stress of these overvoltages. It can also smooth the ripples in the DC current. In addition, in the event of a DC short circuit, the smoothing reactor can reduce the probability of commutation failure by limiting the rapid change of current.